"I am extremely concerned about their health," said their 19-year-old son Harvey. "I am not sure my dad could make it through another year in there."

Over the last decade, Mr Humphrey, a 58-year-old former journalist, and his wife Yu Yingzeng built a reputation as a couple who could investigate the murkiest situations and reveal what was really going on.

Mr Humphrey described his wife, who later took American citizenship, as "probably China's best forensic accountant".

Yu Yingzeng in custody (CNTV)

Their firm, ChinaWhys, specialised in infiltrating companies to sniff out bad apples: the employees bilking their bosses by lining their own pockets or pushing business to their family members.

But Harvey, their only son, had just finished school and was planning to study mechanical engineering back in England. And his parents were going to move back with him.

"They were certainly planning to sell within a year and they were going to move to where I was going to go to university," he said.

Then came the call from GlaxoSmithKline. One of the first Western pharmaceutical companies to enter China, GSK has so far spent £300 million trying to crack what will soon be the world's second-largest drugs market.

But it was under attack from within. An anonymous internal whistleblower had alerted first the main board, and then the police, to alleged corruption in the company's Chinese operations.

Then there was the email to the board which had attached a copy of a sex tape featuring Mark Reilly, GSK's China boss, and his Chinese girlfriend, a secretary at a travel agency used by the company, shot inside his home.

Mr Reilly, who has since been charged with corruption by the Chinese police, wanted to know who the whistleblower was.

He suspected Vivian Shi, who had been hired for her impeccable connections inside the Communist party, but who had recently left the company under a cloud.

Johnson & Johnson, the American pharmaceutical giant, had also come under attack from a whistleblower a few months after Ms Shi had ceased employment there in 2004.

"I am NOT the whistleblower," Ms Shi previously wrote to The Telegraph. "I am the victim of the intensified internal management war between Mark Reilly and John Lepore. (They were the two bosses for GSK China in March 2010 to September 2012)."

By investigating Ms Shi, Mr Humphrey and his wife were entering dangerous waters. Her father had been a senior Communist party cadre in Shanghai and is thought to have been a mentor to Meng Jianzhu, now the Politburo member who runs China's police and legal system.

Peter Humphrey in custody (CNTV)

Shortly after Ms Shi discovered she was the subject of their investigation, dubbed "Project Scorpion", the couple were arrested. A month later, Mr Humphrey found himself paraded on national television, reading out a confession on camera.

Over the past year, confined in a cell with seven other prisoners, he has "become increasingly despondent" according to a family friend.

Conditions are grim. Their diet mainly consists of packets of instant noodles. Once a week, Mr Humphrey is forced to shave with a shared razor. "That is particularly unpleasant," he said in one letter.

The couple have been separated, but have occasionally snatched glimpses of each other through windows or doors. Their son, Harvey, has not been allowed to visit.

"He saw his wife in a corridor on May 29," said a family friend, citing the latest report from consular officials. "She was only about 10ft away but she did not recognise him. He was very upset and felt she looked dazed and disorientated."

For their trial, scheduled on the afternoon of August 7, Mr Humphrey has asked for a neck brace "because his head keeps falling to the left". The request was denied.

"Spine, hernia and prostate treatment have been withheld and he is in constant pain," said the friend, citing the consular officials.

Once a month, consular officials are allowed to collect letters, some of which have been seen by The Telegraph. Unable to write because of arthritis in his hand, Mr Humphrey dictated his letters, in which he detailed his extensive health problems.

Last November, he said was suffering "severe frequent pain" in both legs, and numbness in his right leg. "Seems to be a blood circulation problem," he wrote in November.

Fumes from a rubbish incinerator next to the detention centre may have triggered a series of serious asthma attacks. "We get the smell and smoke coming into our cell and it feels toxic," he wrote.

"All I get now is one sleeping pill and in the morning one so-called pain killer," he wrote in one letter. "But the reality is that I am in constant pain from my legs and joints and my damaged spine, especially the neck vertebrae. And I am not getting any treatment from my prostate problem now or any further investigations into it [...] I need surgery on my hernia. I simply despair over my collapsing health and mummy's."

"Mummy - I have spoken to other prisoners who have seen her and they say she is in constant pain from joint problems caused by her confined conditions. They say her weight has fallen to below 40kg (six stone). I have lost 10kg of body weight and am very skinny."

Mrs Yu also has problems with her kidneys, but medical staff at the detention centre reportedly said they were not overly concerned because "she only needs one working kidney".

Mr Humphrey's roots in China go back to his university days in Durham, where he studied Chinese and was the president of the Far Eastern society and an Oriental film club.

"I helped the first students from China to settle in at university," he wrote in a letter that his son received earlier this week.

"After graduation, I helped China by teaching English to scientists preparing to be sent abroad, and later I was on the founding team of the China Daily (the state-owned English-language newspaper)."

He went on to work for the South China Morning Post and Reuters before leaving Before founding ChinaWhys, Peter had served as China country manager for US risk consultancy Kroll and head of China investigations at PwC as well as undertaking a number of humanitarian assignments.

He became the president of Beijing's Rotary Club. "We saved the lives of over 3,000 Chinese children with heart disease by funding their surgery. We helped build the first private school for autistic children".

The couple have been charged with an invasion of privacy, and despite discussions between David Cameron and Li Keqiang, the Chinese prime minister, about the case, the court will be closed to protect Ms Shi's privacy.

"We got caught up in a war unintentionally," Mrs Yu told consular officials last week. "But I have no grudges." She added that she understood Chinese officials were all doing their job, including the police.

"I hope that my parents will be released very soon after the trial," said Harvey. "I am aware that the consulates have asked that the trial be open for them to attend, but I am confident that they will do their utmost to ensure I can attend with consular officials."

GSK previously said Mr Humphrey had not been an employee of the company. A spokesman said GSK had "hired ChinaWhys in April 2013 to conduct an investigation following a serious breach of privacy and security related to the company's China general manager."

The spokesman declined to comment on accusations that GSK had not provided any subsequent support to Mr Humphrey and his wife.